Coals are extensively utilized as fuels for thermal electric-power generation or boilers or as starting materials for chemical products, and there is a strong desire to develop a technique for efficiently removing the ash matter contained in coals, as a measure for environmental preservation. For example, in a high-efficiency combined electric-power generation system based on gas turbine combustion, an attempt is being made to use an ash-free coal (HPC) from which ash matter has been removed, as a fuel that replaces liquid fuels including LNG. It is also attempted to use an ash-free coal as a feed coal for steelmaking cokes, such as cokes for blast furnaces.
Proposed as a process for producing an ash-free coal is a process in which a solution containing coal components soluble in solvents (hereinafter referred to as “solvent-soluble components”) is separated from a slurry by using a gravitational settling method (for example, JP-A-2009-227718). This process includes a slurry preparation step in which a coal is mixed with a solvent to prepare a slurry and an extraction step in which the slurry obtained in the slurry preparation step is heated to extract solvent-soluble components. This process further includes: a solution separation step in which a solution containing the solvent-soluble components dissolved therein is separated from the slurry in which the solvent-soluble components have been extracted in the extraction step; and an ash-free-coal acquisition step in which the solvent is separated from the solution separated in the solution separation step, thereby obtaining an ash-free coal.
In the extraction step of a conventional process for ash-free-coal production, the slurry obtained in the slurry preparation step is heated to a given temperature and supplied to an extraction tank. The slurry supplied to the extraction tank is held at a given temperature while being stirred with a stirrer, thereby extracting solvent-soluble components. In this extraction step, the slurry is allowed to stay in the extraction tank for about 10-60 minutes in order to sufficiently dissolve the solvent-soluble components in the solvent.
Since the time period required for extracting the solvent-soluble components in the extraction step considerably affects the time period required for ash-free-coal production, there has conventionally been a request for shortening the extraction period. If the time period required for heating the slurry to the given temperature can be shortened the extraction period in the extraction step can be shortened. It is hence possible to shorten the extraction period by rapidly elevating the temperature of the slurry to the given temperature in the extraction step.
It seems that as a method for rapidly elevating the temperature of the slurry to the given temperature, use can be made, for example, of a method in which in the slurry preparation step, a coal is mixed with a preheated solvent so that the slurry to be introduced into the extraction step has a temperature elevated beforehand. However, the higher the temperature of the solvent to be mixed with the coal, the higher the apparatus design pressure and the higher the equipment cost and operating cost. It is hence difficult to rapidly elevate the temperature of the slurry at low cost.